Ducted Fan Pseudo-Jets

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sanman

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I want to ask about Ducted Fan technology, given the recent marketplace entry of a couple of new aircraft, the PJ-2 Dreamer and the UL-39 Albi, both featuring ducted fan propulsion powered by piston engines.




Firstly, what is the scope for development of this technology? What are its limitations?

Nextly, is there any other parlance for this type of aircraft? Pseudo-jets? Quasi-jets? Jet-like? (Anything to help my Google-searching)

So there is Electric Ducted Fan technology, which has developed under the r/c model and drone market.
But with PJ-2 and UL-39 we now see the advent of piston-powered Ducted Fan propulsion -- the key being that reciprocating piston engines are allowable under LSA regulations, whereas turbines are not.

One problem seems to be in power-to-weight, given that you need a powerful piston engine and gear transmission to bring the fans up to useful rpm.

What are the ways to solve these problems, while still falling under LSA restrictions?
 

sanman

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I wanted to ask if something like a Wankel engine might be useful there, since they're more comfortably able to operate at higher rpm.
Wankels of course suffer from their own set of problems, like compression ratio and leaky seals from abrasion wear. But still, Wankel engines are a mature technology
But instead of Corvette power, how about RX-7 power?

 

henryk

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ducted fan propulsion

=to big distance fan tip/duck...(thrust loses !)


=NOTCHED, STEPPED fans=remedium.

+ CRDiff. reductor (with two high RPM motorcycle engines,UL39ALBI like) for circa 30% thrust gain in comparation with one step fan=

+in future E-hybrid ???
 

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galapoola

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I wanted to ask if something like a Wankel engine might be useful there, since they're more comfortably able to operate at higher rpm.
Wankels of course suffer from their own set of problems, like compression ratio and leaky seals from abrasion wear. But still, Wankel engines are a mature technology
But instead of Corvette power, how about RX-7 power?

Moller used a Wankel in a ducted fan configuration in his VTOL project. Never went anywhere except tethered hover.
 

sanman

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=to big distance fan tip/duck...(thrust loses !)


=NOTCHED, STEPPED fans=remedium.
Sounds good - has anyone come out with an actual flying vehicle demonstrating this?

+ CRDiff. reductor (with two high RPM motorcycle engines,UL39ALBI like) for circa 30% thrust gain in comparation with one step fan=

+in future E-hybrid ???
But does LSA allow the use of twin engines?



Moller used a Wankel in a ducted fan configuration in his VTOL project. Never went anywhere except tethered hover.
Jetson is trying to do their electric version with a more stripped down look, like a flying dune-buggy:


But there's no way batteries can beat hydrocarbon fuel.

Some company named Liquid Piston has turned the Wankel inside out, creating what they've dubbed as a "non-Wankel rotary engine"


They're claiming to have solved some of the older Wankel problems, but some remain, like oil going into the crankcase and burning with fuel.
On the other hand, they have gotten a hydrogen version running, which doesn't have that problem either. Hydrogen is not only zero-emissions, but it can also provide savings on fuel load.

Another engine you may have heard of is the opposed piston engine from Achates Power:


It claims 50% greater efficiency improvement, but no word on when anything will be mass-produced.
 

Riggerrob

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The Greek-designed Archon Sf-1 will soon be available - in kit form - from Fisher Flying Products in the USA.
The Greek designers are also working on a 2-seater Archon SF-2 variant.

Another rumor has it that Bart VerHees has lost interest in marketing Verhees Delta 2 kits. They are currently seeking a partner to produce kits.
 

Malish

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But does LSA allow the use of twin engines?
PJ-II "Dreamer" is single engine aircraft with twin ducted fans:
 

Riggerrob

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look up the Rohr 2-175 fan-jet. Rohr industries specializes in building jet engine nacelles for airliners. When they had some spare R&D money they built a pair of pretty little, two-seater deltas with ducted fans. Unfortunately, Rohr lost money in other parts of their business, so were forced to cancel the 2-175 project after a bunch of test flights. Pity!
 

Vigilant1

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For most applications, it would make zero sense to try to buy/make a highly efficient engine just to mate it to a highly in-efficient (compared to an open prop) ducted fan.
If someone wants a ducted fan, it likely makes more sense to accept the inefficiency that goes with it. Find an engine that can produce the power you need at a price you are willing to pay. Engine efficiency will likely be far less important than power-to-weight and (maybe) power-to-cost.
Malish appears to understand all of this very well and he and his team are building a power package and airframe that works well within the constraints imposed by the desire to use a ducted fan.
 

sanman

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f.e. RED3 engine (CELERA 500L) is 2 x 6 cyllinders ...
I'd seen videos on Celera500L, but haven't heard much about it lately. I thought it was originally supposed to be a V8 engine.

The Greek-designed Archon Sf-1 will soon be available - in kit form - from Fisher Flying Products in the USA.
The Greek designers are also working on a 2-seater Archon SF-2 variant.

Another rumor has it that Bart VerHees has lost interest in marketing Verhees Delta 2 kits. They are currently seeking a partner to produce kits.
But Archon is a pusher-prop, not a ducted fan. Right now it just has the cool looking military jet style airframe, without anything close to jet propulsion.
It would be cool if they eventually came out with a twin ducted-fan version for it, just like with PJ-2 Dreamer.
I happen to like Archon's rectangular (faux) "inlets" over PJ-2's round ones (sort of like how I like modern car headlights over old-style rounded ones)


PJ-II "Dreamer" is single engine aircraft with twin ducted fans:
Okay, so it has a single engine/powerplant, even while coupling that to twin ducted fans. I presume that single engine is what meets the LSA criteria.
 

sanman

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Rohr 2-175 fan-jet
Wow, that thing looks more like a Star Wars Imperial Shuttle than like a mainstream aircraft.
I think that overly exotic looks tend to turn people off, and make it look like a niche pursuit.

For most applications, it would make zero sense to try to buy/make a highly efficient engine just to mate it to a highly in-efficient (compared to an open prop) ducted fan.
If someone wants a ducted fan, it likely makes more sense to accept the inefficiency that goes with it. Find an engine that can produce the power you need at a price you are willing to pay. Engine efficiency will likely be far less important than power-to-weight and (maybe) power-to-cost.
Malish appears to understand all of this very well and he and his team are building a power package and airframe that works well within the constraints imposed by the desire to use a ducted fan.
I've heard that Fan Jets are unusually loud. Ducted Fans would at least muffle some of that noise. We should recognize that other factors like noise count too, and not just efficiency.

Ducted fans are most efficient at slow airspeeds (think tuboat).
With ducted fans do we have to think in terms of Reynolds number instead of Mach number, to describe flow conditions and energy losses?
I took chemical engineering in university, so we were used to calculating flow conditions inside of pipes, ducts, etc more than for unbounded flows.
 

sanman

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I thought thrust force is imparted to fan blades as they push air past them.
Or are you saying that the exhaust-facing side of the fan's hub can be shaped to improve the exhaust flow?

Regarding your slow-motion video, I guess it shows how air flow is directed toward the center-line of thrust. Is slipstream then a de facto boundary?

Btw, is it possible to do afterburner for Ducted Fan? I think it should be, since afterburner just means dumping fuel aerosol into exhaust stream. In the case of turbine, that fuel aerosol will of course naturally ignite due to high temperature. But for ducted fan, then we would need ignition source (perhaps ignition source could be located in EJECTOR Nacelle?)

Hehe, or maybe we can make fake "afterburner" by putting bright lights inside of duct ;) (on EJECTOR nacelle)
We divert engine exhaust out through fan duct, and we have lightbulb inside duct to light up smoke in nice bright orange color 😁
(No, wait - make it neon blue color - then even real fighter pilots will be jealous)
 
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Dan Thomas

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The Miller JM-2 started out looking like this:

1637882445990.png

Then it looked like this:

1637882488962.png

It performed better with an open prop than with the ducted fan.
 
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